We have studied the interaction between lipid bilayer vesicles and cells, using three cell types: human peripheral blood lymphocytes, human red blood cells, and a mouse tumor cell line (P388). As markers for the membrane of the lipid vesicle we used radiolabelled phospholipids and fluorescent lipid probes. As markers for the aquaeous compartment of the vesicle we use radiolabelled sucrose of inulin, and the polar fluorescent dye carboxyfluorescein. We assessed the modes of association of the fluorescent markers with the cells using fluorescence microscopy, fluorometry, flow microfluorometry and fluorescence photo bleaching. Carboxyfluorescein was transferred from vesicle to cell, and free to circulate in the cells' cytoplasm. Sucrose, however, remained in vesicles, which were adsorbed to the cell surface. Fluorescent lipid probes associated with vesicles did not spread out over the cell surface, but remained immobile on the cell surface. A model for vesicle-mediated transfer is proposed in which carboxyfluorescein is released from the vesicles at the cell surface, and subsequently enters the cell.